>It requires knowing your foreground and background colors at render time, which is an unnatural requirement when you want to do GPU-accelerated compositing

Most of the apple hardware supports ARB_blend_func_extended or even EXT_framebuffer_fetch, which lets you to implement subpixel blending with ease. Otherwise you just have to fall back to traditional alpha blending and render each glyph in three passes for each color channel, which is not that hard either.